It is well known that in order to obtain the optimum reinforcement properties imparted by a filler, it should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. Such conditions may only be obtained insofar as the filler has a very good ability, to disagglomerate and be incorporated into the matrix during mixing, and to be dispersed homogeneously in the elastomer.
It is well known that carbon black has such abilities, which is generally not true of inorganic fillers. For reasons of mutual attraction, the inorganic filler particles have an irritating tendency to agglomerate together within the elastomeric matrix. These interactions have the harmful consequence of limiting the dispersion of the filler, causing the reinforcing properties to become substantially lower level than that which it would be theoretically possible to achieve if all the (inorganic filler/elastomer) bonds that could be created during the mixing operation were in fact obtained. These interactions tend to increase the consistency of the rubber compositions in the uncured state and to make them more difficult to work (“processability”)than in the presence of carbon black.
Since fuel economies and the need to protect the environment have become priorities, it has become desirable to produce tires having reduced rolling resistance, without adversely affecting their wear resistance. This has been made possible in particular due to the discovery of new rubber compositions reinforced with specific inorganic fillers, referred to as “reinforcing” fillers. These organic fillers are comparable to conventional carbon black from the reinforcing point of view. They also impart to the rubber compositions a low hysteresis, which is synonymous with lesser rolling resistance for the tires comprising them.
Such rubber compositions, comprising reinforcing inorganic fillers of the siliceous or aluminous type, have for example been described in the following patents or patent applications EP-A-0501227 (or U.S. Pat. No. 5,227,425), EP-A-0735088 (or U.S. Pat. No. 5,852,099), EP-A-0810258 (or U.S. Pat. No. 5,900,449), EP-A-0881252, W099/02590, W099/02601, W099/02602, W099/28376, WO00/05300 and WO00/05301.
In particular, European Patent Applications, EP-A-0501227, EP-A-0735088, and EP-A-0881252, disclose diene rubber compositions reinforced with precipitated silicas of high dispersibility. Such compositions make it possible to manufacture treads having a significantly improved rolling resistance, without adversely affecting grip, endurance and wear resistance. Compositions having this combination of properties are also described in applications EP-A-0810258 and WO99/28376, with specific aluminous fillers (alumina or aluminium oxide-hydroxides) of high dispersibility as reinforcing inorganic fillers, or alternatively in applications WO00/73372 and WO00/73373, which describe specific titanium oxides of the reinforcing type.
Although the use of these specific, highly dispersible inorganic fillers as reinforcing fillers, has reduced the difficulties of processing the rubber compositions that contain them, they are still more difficult to process than rubber compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also known as a bonding agent. The coupling agent functions to provide the connection between the surface of the inorganic filler particles and the elastomer, while facilitating the dispersion of this inorganic filler within the elastomeric matrix.
The term (inorganic filler/elastomer) “coupling” agent is understood to mean an agent capable of establishing a sufficient chemical and/or physical connection between the inorganic filler and the diene elastomer. Such a coupling agent, which is at least bifunctional, has, for example, the simplified general formula “Y—W—X”, in which:
Y represents a functional group (function “Y”) which is capable of bonding physically and/or chemically with the inorganic filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, surface silanols in the case of silica);
X represents a functional group (function “X”) which is capable of bonding physically and/or chemically with the diene elastomer, for example by means of a sulphur atom;
W represents a divalent group linking Y and X.
The coupling agents should not be confused with simple agents for covering the inorganic filler. These simple agents may comprise the Y function which is active with respect to the inorganic filler, but are devoid of the X function which is active with respect to the diene elastomer.
Coupling agents, in particular (silica/diene elastomer) coupling agents, have been described in numerous documents. The most exemplary example of these coupling agents are bifunctional alkoxysilanes.
Patent application FR-A-2094859 discloses the use of a mercaptosilane coupling agent for the manufacturing of treads for tires. It was quickly discovered and is well known today that the mercaptosilanes, and in particular γ-mercaptopropyltrimethoxysilane or γ-mercaptopropyltriethoxysilane, are capable of giving excellent silica/elastomer coupling properties. However, these coupling agents cannot be used industrially because of the high reactivity of the —SH functions, which very rapidly results in premature vulcanization during the preparation of the rubber composition in an internal mixer, producing very high Mooney plasticity values, also known as “scorching”. The resulting rubber compositions are virtually impossible to work with and process industrially. To illustrate the difficulty of using such coupling agents in rubber compositions that contain them industrially, the following documents, FR-A-2206330 and U.S. Pat. No. 4,002,594 may be referred to for guidance.
To overcome this drawback, these mercaptosilanes may be replaced by polysulphurized alkoxysilanes, in particular bis-(C1-C4)alkoxysilylpropyl polysulphides, as described in numerous patents or patent applications (see for example FR-A-2206330, U.S. Pat. No. 3,842,111, U.S. Pat. No. 3,873,489, U.S. Pat. No. 3,978,103 and U.S. Pat. No. 3,997,581).
These polysulphurized alkoxysilanes are generally considered to be the products which, in the case of vulcanized rubber compositions filled with silica, provide the best compromise in terms of resistance to scorching, processability and reinforcing power. Among these polysulphides, mention must be made in particular of bis-3-triethoxysilylpropyl tetrasulphide (abbreviated to TESPT), which is the most commonly used coupling agent in rubber compositions for tires based on a reinforcing inorganic filler. However, this compound is relatively costly and is required in a relatively large quantities.